Optical systems used in binoculars have optical paths folded by means of reflective prisms and are arranged to give erect images. The substitution of prisms by mirrors has been analysed in a paper "Camera viewfinder using tilted concave mirror erecting elements" by Donald DeJager in the SPIE Vol 237 at page 292 of 1980 but the arrangement was found unsatisfactory due to excessive amounts of astigmatism, variation of focus across the field, distortion and coma. Arrangements using erecting mirrors are also described in U.S. Pat. Nos. 4,598,981, 4,804,258, 4,758,077, 4,221,459 and 3,897,133. The first two do not incorporate any lenses and solely use mirrors which do not have an erecting function. The last three have circular fields and apertures with the last one having an annular aperture and it is known that annular apertures give poor visual images.